In order to detect irregularities during the operation of a wind turbine and to trigger an alarm condition based upon the detected irregularity, wind turbine monitoring systems of the state of the art obtain measurement values of various kinds and evaluate the values thus obtained to determine if a predefined alarm condition is satisfied.
The system proposed in European patent specification EP 1 531 376 B1 begins to capture input measurement values whenever predetermined operation variables of the wind turbine are within a certain predetermined operation range, said operation variables being received from the control system of the wind turbine. Such taking of data upon occurrence of predetermined conditions is often described as triggered capturing or triggered measurements.
However, as data capturing commences after a pre-specified triggering condition has been met, the capturing of data may suffer from the incoming data becoming unstable during the intended capturing time. Such instability of the captured values may involve sudden changes of the operating parameters of the wind turbine. The system proposed in the aforementioned European patent specification detects if any of the measured operation variables varies during the entire signal capturing beyond a predefined upper or lower limit, and, if so, aborts the capturing and discards the captured data. A consequence of this is that the capturing procedure must be started again as soon as the triggering condition is met again and further that valuable information is lost by discarding the recorded data.
United States patent application publication US 2003/0200014 A1 discusses the need for different alarm thresholds depending on the operating parameters of the monitored device. A similar observation is made regarding wind turbines in Orbit 2Q04, p. 21-27.